The present invention relates in general to binoculars and in particular to a new and useful drive arrangement for zoom type binoculars which include a lever which can be operated to change the magnification of the binoculars.
Reference is now made to FIG. 1 which is a front elevational view of conventional zoom type binoculars showing the zoom (gear) mechanism. The binocular body comprises a pair of body portions 50 and 50' which are pivotally mounted to each other. A central gear 20 is rotatably mounted near or at the pivot axis between body portions 50 and 50'. Central gear 20 is directly rotated by a pivotally mounted operation lever 21 connected to the central gear 20. Left and right tube gears 30 and 30' are provided which are connected to respective left and right lens tubes. Tube gears 30, 30' are adapted to be driven by central gear 20 through intermediate gears 40, 40', respectively. An adjustment of magnification is thus provided in known fashion.
In the case of zoom binoculars of this type, which include an operating lever disposed at the center of the binocular body, the rotation angle of the lever is usually limited to about 60.degree.. For this reason, a sufficiently large rotation of tube gears 30, 30' cannot be obtained unless the diameter of the central gear 20 is large. The central gear 20 must, therefore have a large diameter.
This, however, raises another problem in the overall operation of the zoom binocular. Specifically, with the central gear 20 assumed to be fixed, the intermediate gear 40 is rotated to a large extent in the direction of arrow A when the distance between the two lens tubes is adjusted to meet varying distance between the eyes of different users. This adjustment is illustrated by the dot-dash line in FIG. 1 which indicates the position of the left body portion 50 after it has been moved in the direction of arrow B. With the movement of body portion 50 in the direction of arrow B, and the resulting rotation of intermediate gear 40 in the direction of arrow A, tube gear 30 is rotated counterclockwise to cause a corresponding change in magnification on the left side of the binocular. This increases the difference of magnification between the two lens tubes and the two sides of the binocular.
An increased size of the central gear 20 which is required also raises problems in operating the zoom binoculars in that the larger diameter gear causes a greater resistance in the manipulation of the operating lever 21.